Warfarin (coumarin) is an anticoagulant that acts by inhibiting vitamin K-dependent coagulation factors. Warfarin based compounds are, typically, derivatives of 4-hydroxycoumarin, such as 3-(α-acetonylbenzyl)-4-hydroxycoumarin (COUMADIN). COUMADIN and other coumarin anticoagulants inhibit the synthesis of vitamin K dependent clotting factors, which include Factors II, VII, IX and X. Anticoagulant proteins C and S are also inhibited by warfarin anticoagulants. Warfarin is believed to interfere with clotting factor synthesis by inhibiting vitamin K epoxide reductase, thereby inhibiting vitamin K regeneration.
An anticoagulation effect is generally seen about 24 hours after administration of a single dose of warfarin and is effective for 2 to 5 days. While anticoagulants have no direct effect on an established thrombus and do not reverse ischemic tissue damage, anticoagulant treatment is intended to prevent the extension of formed clots and/or to prevent secondary thromboembolic complications. These complications may result in serious and possibly fatal sequelae.
The FDA has approved warfarin for the following indications: 1) the treatment or prophylaxis of venous thrombosis and pulmonary embolism, 2) thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement, and 3) reducing the risk of death, recurring myocardial infarction, and stroke or systemic embolism after myocardial infarction.
A number of adverse effects are associated with the administration of warfarin. These include fatal or nonfatal hemorrhage from any tissue or organ and hemorrhagic complications such as paralysis. Other adverse effects include paresthesia including feeling cold and chills; headache; chest, abdomen, joint, muscle or other pain; dizziness; shortness of breath; difficult breathing or swallowing; unexplained swelling, weakness, hypotension, or unexplained shock. Other adverse reactions reported include hypersensitivity/allergic reactions, systemic cholesterol microembolization, purple toes syndrome, hepatitis, cholestatic hepatic injury, jaundice, elevated liver enzymes, vasculitis, edema, fever, rash, dermatitis, including bullous eruptions, urticaria, abdominal pain including cramping, flatulence/bloating, fatigue, lethargy, malaise, asthenia, nausea, vomiting, diarrhea, pain, headache, dizziness, taste perversion, pruritus, alopecia and cold intolerance.
Drug toxicity is an important consideration in the treatment of humans and animals. Toxic side effects resulting from the administration of drugs include a variety of conditions that range from low grade fever to death. Drug therapy is justified only when the benefits of the treatment protocol outweigh the potential risks associated with the treatment. The factors balanced by the practitioner include the qualitative and quantitative impact of the drug to be used as well as the resulting outcome if the drug is not provided to the individual. Other factors considered include the physical condition of the patient, the disease stage and its history of progression, and any known adverse effects associated with a drug.
Drug elimination is the result of metabolic activity upon the drug and the subsequent excretion of the drug from the body. Metabolic activity can take place within the vascular supply and/or within cellular compartments or organs. The liver is a principal site of drug metabolism. The metabolic process can be broken down into primary and secondary metabolism, also called phase-1 and phase-2 metabolism. In phase-1 metabolism, the drug is chemically altered by oxidation, reduction, hydrolysis, or any combination of the aforementioned processes and usually yields a more polar product than the parent drug. In Phase-2 metabolism the products of the phase-1 reaction are combined with endogenous substrates, e.g., glucuronic acid, to yield an addition or conjugation product that is even more hydrophilic than the product of phase-1 and which is readily eliminated in the bile or in the urine. In some cases, a drug can undergo only phase-2 (conjugation) metabolism, in other cases a drug can be eliminated unchanged. The first step in such synthetic reactions is often an oxidative conjugation performed by the cytochrome P450 (CYP450) system. Metabolites formed in phase-2 reactions are typically the product of a conjugation reaction performed by a transferase enzyme. These reactions include glucuronidation, amino acid conjugation, acetylation, sulfoconjugation, and methylation.
Mammalian cytochrome P450 enzymes (CYP450), including human CYP450, are membrane-bound heme-containing proteins that were originally discovered in rat liver microsomes. In order to function, CYP450 enzymes need a source of electrons. There are two different kinds of electron transfer chains for CYP450s. These depend on the location of the enzyme in the cell. Some P450s are found in the mitochondrial inner membrane and some are found in the endoplasmic reticulum (ER). The protein that donates electrons to CYP450s in the ER is called NADPH cytochrome P450 reductase. Ferredoxin is the immediate donor of electrons to the CYP450s in mitochondria (CYP11A1, CYP11B1, CYP11B2, CYP24, CYP27A1, CYP27B1, CYP27C1). NADPH is the source of electrons that flow from ferredoxin reductase to ferredoxin and then to CYP450. A few P450s also can accept electrons from cytochrome b5.
Polymorphisms (differences in DNA sequence found at 1% or higher in a population) can lead to differences in drug metabolism, so they are important features of CYP450 genes in humans. CYP2C19 has a polymorphism that changes the enzyme's ability to metabolize mephenytoin (a marker drug). In Caucasians, the polymorphism for the poor metabolizer phenotype is only seen in 3% of the population. However, it is seen in 20% of the Asian population. Because of this difference, it is important to be aware of a person's race when drugs are given that are metabolized differently by different populations. Some drugs that have a narrow range of effective dose before they become toxic might be overdosed in a poor metabolizer.
CYP2D6 is perhaps the best studied P450 with a drug metabolism polymorphism. This enzyme is responsible for more than 70 different drug oxidations. Since there may be no other way to clear these drugs from the system, poor metabolizers may be at severe risk for adverse drug reactions. CYP2D6 Substrates include antiarrhythmics (Flecainide, Mexiletine, Propafenone), antidepressants (Amitriptyline, Paroxetine, Venlafaxine, Fluoxetine, Trazadone), antipsychotics (Clorpromazine, Haloperidol, Thoridazine), beta-blockers (Labetalol, Timolol, Propanolol, Pindolol, Metoprolol), analgesics (Codeine, Fentanyl, Meperidine, Oxycodone, Propoxyphene), and many other drugs. CYP2E1 is induced in alcoholics. There is a polymorphism associated with this gene that is more common in Chinese people.
The CYP3A subfamily is one of the most important drug metabolizing families in humans. CYP3A4 is “the most abundantly expressed CYP450 in human liver”. (Arch. Biochem. Biophys. 369, 11-23 1999) CYP3A4 is known to metabolize more than 120 different drugs, e.g., acetaminophen, codeine, cyclosporin A, diazepam, erythromycin, lidocaine, lovastatin, taxol, cisapride, terfenadine, and warfarin, to name a few.
The number of adverse drug reactions (ADRs) in the United States has risen dramatically in recent years and now represents a critical national health problem. The World Health Organization (WHO) defines an ADR as “a response to a drug that is noxious and unintended and occurs at doses normally used in man for the prophylaxis, diagnosis or therapy of disease, or for modification of physiological function”. To highlight the importance of error in the genesis of ADRs and the fact that most (30-80%) ADRs are preventable, a more recent definition of an ADR is “an appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product, which predicts hazard from future administration and warrants prevention or specific treatment, or alteration of the dosage regimen, or withdrawal of the product.”
Because ADRs are a major source of morbidity and mortality in our health care system, reducing the incidence of ADRs has become a national priority (FDA, Center for Drug Evaluation and Research). According to formal estimates, greater than 2.5 million ADRs occur each year in hospitals, ambulatory settings and nursing homes, resulting in over 106,000 deaths, and costing the US economy $136B annually in drug-related morbidity and mortality. This expense is greater than the annual cost of cardiovascular disease and diabetes in the United States. In addition, the estimated mortality rate associated with ADRs make them the fourth leading cause of death in this country.
Many ADRs arise from the fact that most drugs developed by the pharmaceutical industry significantly interact with components of the CYP system, either by relying on them for their metabolism and/or by inhibiting or inducing various CYP fractions. In other words, because so many important drug classes (e.g., antihypertensives, antihistamines, antidepressants, immunosuppressants, statins) interact with the CYP system, it can act as a “bottleneck” for the safe metabolism and elimination of these agents and lead to toxic effects. With regard to drug metabolism, two fractions of the CYP system merit special mention: CYP3A4 and CYP2D6. Approximately one half of all known drugs interact with CYP3A4. Likewise, CYP2D6, an enzyme fraction whose activity is highly dependent on genetics (genetic polymorphisms), metabolizes one third of drugs in clinical use. Both of these enzymes are involved in the metabolism of warfarin-like compounds.
The vast majority (70-90%) of ADRs occur as extensions of their expected pharmacological effects (exaggerated pharmacology). This is particularly relevant to the use of warfarin since the extension of the warfarin pharmacological effect is bleeding. Although many different factors can contribute to the development of ADRs, altered drug metabolism leading to elevated drug levels, either due to drug interactions at the enzymatic level, genetic alterations in enzyme activity, and/or organ dysfunction (liver, kidney), play a particularly important role in the genesis of ADRs.
Drug therapy using warfarin is particularly difficult because the metabolism of warfarin is complex and subject to interactions with a host of other drugs, including drugs that are commonly prescribed in patients suffering from atrial fibrillation, such as amiodarone for example. Warfarin is a mixture of enantiomers having different intrinsic activities at the vitamin K epoxide reductase (VKER) enzyme. These enantiomers have different metabolic pathways using different CYP450 isozymes. The CYP450 metabolic system is highly inducible or repressible by a host of external factors such as diet and other medications. Also, the CYP450 system is subject to many genetic variations and has a low capacity and is easily saturable. For these reasons the metabolism of warfarin is subject to unpredictable variations and each enantiomer has a different metabolic fate and different potencies at the VKER enzyme.
In addition, warfarin activity at the VKER enzyme results in inhibition of coagulation factors II, VII, IX, and X, which have different half-lives of their own, ranging from hours (factor VII) to days (factor X). Because of this complex situation, the pharmacological effect (increased coagulation time) of warfarin becomes apparent only 5 to 10 days after a dose. It is therefore easy to understand why warfarin therapy is extremely difficult to predict and why patients are at high risk of bleeding complications including death. In the current state of warfarin therapy, patients on warfarin must report to a coagulation lab once a week in order to be monitored and in order to detect any early risk of bleeding complications. Even with this strict monitoring system, many patients on warfarin die every year from bleeding complications.
The potential clinical problems and business risk associated with developing drugs, which must past through the P450 metabolism “gauntlet”, is markedly increased in the United States by the following two facts: 1) the number of prescriptions filled in this country has increased to about 3 billion per year or 10 per person, and 2) patients, particularly those that live longer and have more complex medical problems, tend to take multiple medications. The latter issue is important because the incidence of ADRs rises exponentially when subjects take more than four drugs. Although it is good practice to avoid polypharmacy, in many cases this is not possible because patients require different classes of drugs to effectively treat complex medical conditions.
The landscape of drug R&D is littered by failed drugs that were withdrawn by the FDA because they caused fatal ADRs involving CYP metabolism. These drugs were clinically effective and in many cases commercially successful. Notable drugs that were withdrawn due to ADR-related deaths involving CYP450 metabolism include terfenadine (February 1998), astemizole (July 1999) and cisapride (January 2000). In each of these cases, drug interactions involving CYP3A4 caused concentrations of the pharmaceutical agent to increase to such a degree that it significantly inhibited a particular type of potassium channel in the heart called IKr, which in turn, prolonged the QT interval and caused a potentially fatal form of ventricular tachyarrhythmia called torsades de pointes.
A warfarin analog that has a controllable and a predictable metabolic fate, not depending on CYP450, is therefore highly desirable and would be an important addition to the armamentarium of drugs available for treating atrial fibrillation patients. Certain warfarin analogs have been previously reported. See, for example, WO 02/085882, which is incorporated herein by reference.